Firstly, there is the Bitcoin Core client, which is the "reference implementation". There are other implementations/clients as well.

The Bitcoin Core client is developed to follow the Bitcoin protocol but also has features like a wallet, a GUI, etc.

So there are two facets to consider:

There are some articles on the Bitcoin Wiki that will be of interest here:

Development process https://en.bitcoin.it/wiki/Development_process

Release process https://en.bitcoin.it/wiki/Release_process

And there's this, in the bitcoin repo:

Developer Notes https://github.com/bitcoin/bitcoin/blob/master/doc/developer-notes.md

There was also a session at Bitcoin 2020 conference that might be of interest:

Bitcoin Core Development Process - Bitcoin 2022 Conference

Then there are second layer protocols and applications -- e.g., Lightning network. Those have their own methods for developing the protocol and app/features.

I personally don't need to convince the developers to include or not include something in a release. They need to convince me that I want to install their release.

Otherwise, their change can do whatever, even things that might be harmful, but that doesn't affect me because I'm not running their software. However, if there's a release that is contentious, then I want to be on the side that the vast majority of the "economic nodes", otherwise I may accept payment that I cannot then spend because my payment will not be accepted by those nodes.

So the tl;dr to your question is: Who can change bitcoin? Those who understand Incentives and Game Theory such that a Bitcoin Core release includes only changes that will be accepted by the vast majority of economic nodes.